The present invention relates to readily dispersible granulated material composed of zinc oxide and precipitated silica, a method of its production and its use.
Given the growing awareness of society of the problems relating to the environment and the increasing demand for safe conditions in the work place, the rubber industry, just as other industries, is falling back on the use of starting materials which are dust-free or only very slightly dusty. The requirements placed on the manufacturer of raw products for being able to offer them in a low-dust form are becoming more and more stringent. Another concern of the rubber-processing industry relates to the use of raw materials which can be readily transported and stored in silos. Such raw materials clearly simplify handling since the raw materials can be automatically supplied as silo material with the appropriate transport capabilities for delivery to the mixing units used in the rubber industry. Powdery raw materials can not be readily transported as a rule since the conveying systems become clogged. One solution generally offered is to provide the raw materials in granulated form. However, the good handling properties which can be achieved by granulation are frequently offset by the poor dispersing behavior of this granulated material when compounded with rubber. This poor dispersing behavior is automatically associated with unfavorable mechanical properties (tensile strength, elongation at rupture, tear resistance, etc.) of the finished rubber article.
Zinc oxide belongs to the large groups of compounds frequently used in the rubber industry. Although it can be granulated according to customary methods, it is apparent that the granulated zinc oxide material obtained in this manner can not be dispersed to a satisfactory degree in the rubber mixture. This failing is due to the hardness of the zinc oxide. Another possibility of converting zinc oxide into a low-dust form is presented by the process of pelletizing, whereby zinc oxide is agitated in appropriately equipped rotating drums which brings about a compaction or densification to form spherical pellets of zinc oxide. However, the pellets produced in this manner are very soft and disintegrate readily. Therefore, storage in a silo, and especially pneumatic transport systems for conveying the product, is not possible The dust content is also very high, so that the goal of a low-dust product is achieved only to an insufficient extent. Therefore, even this form of use was not able to find general acceptance in the rubber industry.